The effects of an asteroid or comet impacting the earth depends on the size and composition of the
impactor. There is literally tons of material hitting the earth every day! Most of it is just dust and
burns up in the atmosphere as shooting stars. On October 9, 1992, a fireball was seen streaking
across the sky from Kentucky to New York. At least 14 people captured part of the
fireball
on videotape. Larger pieces may survive their fiery passage through the atmosphere to hit the
ground. It is believed that a
10km
wide rock hitting the earth 65 millions of years ago was responsible for the extinction of the
dinosaurs.

Astronomers know which comets and asteroids pass by us by observing the path or orbit these
objects take around the sun. Just like the planets and moons, asteroids and comets have orbits
which can be mathematically calculated based on observations. These elliptical (egg-shaped) orbits
are relatively unique. When a new asteroid or comet is discovered, astronomers have to observe it
enough times to be able to determine the orbit. There are certain numbers they use to define the
orbit and we call those numbers 'orbital elements.' We can use the orbital elements to predict the
future positions of comets and asteroids within the solar system. All of the asteroids and comets
that we currently know of have orbits that keep them in the solar system so they never really leave
'our space.'

Although we would like to (in theory) make this website the most complete website on comets, we
recognize our limitations. Plus there are already several very good sites on comets in general. A
good place to start is the SEDS Comets page.
Details about comet missions can be found
here.
Another NASA site that has lots of information and pictures of comets is the
Comet Observation Home Page.
For some interesting activities try
Amazing Space's Comets page.
Finally, there is Comets & Meteor Showers.

Good question. Water vapor in the vacuum of space and when exposed to sunlight further breaks down in a process called
photodissociation, into H atoms and OH molecules. The OH molecules fluoresce in the presence of sunlight and is detected
with spectrometers sensitive to Ultraviolet light. In fact, we can measure the abundance of water in a comet's nucleus by
measuring the intensity of the emission bands and applying some scaling factors.

Indeed, one of the products of water dissociation is OH, which produces emission bands in the near-ultraviolet portion of the
spectrum by fluorescence. As seen from Earth, these bands would be expected to brighten following the projectile's impact
with Tempel 1 similar to changes in any species' emission bands due to a change in the amount of gas in the coma. To first
approximation, the intensity of the band is simply proportional to the number of molecules available to fluoresce. Whether the
increase due to the impact is detectable depends on how much new water is dissociated compared with the on-going release
of water from the rest of the comet's surface.(Thanks to Dr. David Schleicher of Lowell Observatory, Flagstaff, AZ)

"Not likely - that phenomenon should not be significantly different from normal comets,
for which there is no indication of lightning. The ionization happens so far from the nucleus that the voltage gradient is tiny. The
separation between electrons and positive ions in the outer coma is relatively small, so that the overall coma is electrically neutral.
Recombination doesn't happen because the density is so low, which also inhibits the electrical conductivity even if the material were
totally ionized."

Prof. Chris Russell of UCLA's Institute of Geophysics and Planetary Physics and PI for the
DAWN mission responds... "Lightning on Earth arises
due to the charging up of water droplets and the separation of droplets of different size (and therefore charge) by the convection of
droplets upward in clouds. Thus gravity in maintaining an atmospheric pressure gradient is also a needed factor in terrestrial
lightning generation. Since on comets gravity is weak, and the pressure and temperature of the gas is not right for water droplet
formation we do not expect lightning to occur. However, nature often surprises us. So one should be vigilant."

Actually, early cultures where not even aware of comets as being icy bodies in the Solar System. So every comet they saw was a
'new' demon in their belief system. It wasn't until the late 1600's that astronomers thought that some comets that had been
observed through the centuries might be the same comets seen over and over. In fact, the "first" comet that is defined as periodic,
meaning that it is seen on a regular basis, is comet Halley. It is named after Edmund Halley who noticed that every 76 years there
was a comet visible. He was able to predict when that one comet would return. Unfortunately, he died before he could see his
prediction come true, but the comet was still named after him. How was he able to predict the return of this comet? Well, every
comet has a very distinctive orbit or elliptical path around the sun. That orbit can be defined with a set of numbers that we refer to as
"orbital elements." Since the path of the comets (and the planets) are in three dimensional space, we need at least 3 numbers to orient
that path, a few others to define the size and shape, and a few to define the position in the orbit. Using these orbital elements, we
can then calculate a comet's past and future positions to predict when it will next be observable. If a comet is found by chance, one
has to observe it long enough to trace its path on the sky, fit it to an ellipse to determine the orbital elements, then compare those
values to a table of known comet orbital elements to identify it.

The nucleus of the comet is its solid part that is embedded in a cloud of gas and dust
called the coma. The term arises from our perception of the comet as an observational phenomenon, of a fuzzy ball of light
with a tail. The nucleus would then be the center of the phenomenon. Until 1986 when the Giotto spacecraft flew past Halley's
comet, we had never seen the nucleus of any comet.

Now that we can get close to a comet and study it in more detail, we start to talk
about the parts of a nucleus going into its interior. We ask if there is a 'crust' of the nucleus which is the top layer that has
been processed by devolatilization (the removal of icy materials). Is there a 'mantle', which is a layer that is denser than the
crust. Since the layering implies some processes involving heat and alteration of material, we then can hypothesize a cometary 'core', which is the very
center of the solid part of the nucleus. We don't know the nature of the crust, mantle or core of the comet or even if they
exist. In the Deep Impact mission we plan on finding out if layering exists and if it does, to study the crust and mantle of the
comet.